Gastrulation and Neurulation

Question 1

What is the form of the disk at the end of week 2?

Answer 1

Bi-laminar disc (epiblast and hypoblast)

Question 2

What happens at the beginning of week 3?

Answer 2

Gastrulation begins

Question 3

What are the key structures on the bi-laminar disc as gastrulation begins?

Answer 3

+ Anterior visceral endoderm of hypoblast defines anterior

+ Primitive streak (groove on epiblast)

+ Primitive pit and associated primitive node

Question 4

What happens at the streak and node during gastrulation when the endoderm is formed?

Answer 4

Epiblast cells stream into the embryo along the primitive streak/node and replace hypoblast cells

Question 5

What happens during gastrulation when the mesoderm is being formed?

Answer 5

+ Epiblast cells stream into the embryo along the primitive streak between the two layers

+ In front of the node a specialised mesoderm structure called the notochord forms between the two layers

Question 6

What is the notochord?

Answer 6

+ An important signalling structure

+ Primitive beginning to the spine

+ Appears in embryos as a small flexible rod made from mesoderm cells

Question 7

What is neurulation?

Answer 7

+ The folding process in vertebrate embryos, - transformation of the neural plate into the neural tube

+ The embryo at this stage is termed the neurula

Question 8

What does the primitive node/notochord organise?

Answer 8

Dorsal structures

Question 9

Why is the notochord an important signalling structure?

Answer 9

It releases factors (chordin, noggin, follistatin) which block the effects of BMP, allowing neural tissue to form instead of skin

Question 10

What happens to the neural plate at around 19 days?

Answer 10

+ Neurulation induced by notochord (deep to neural epithelium)

+ Epithelial cells become columnar

+ The plate will form a tube (neurulation)

+ Day 19+ a midline groove becomes apparent

Question 11

What happens around 20-21 days (end of third week)?

Answer 11

+ Cells on plate edge thicken and form neural folds and a neural groove

Question 12

What happens during day 22+ of the fourth week?

Answer 12

There is fusion in the cervical region of the neural folds and neural tube beings to form

Question 13

What causes the curvature of the columnar cells to form the tube?

Answer 13

Actin rings around apex of columnar cells constricts

Question 14

Approx. how long does it take for the rostral/nose-end to close?

Answer 14

25 days

Question 15

What forms at the rostral/nose-end?

Answer 15

rostral neuropore

Question 16

Approx. how long does it take for the caudal/tail-end to close?

Answer 16

27 days

Question 17

What forms at the caudal/tail-end?

Answer 17

Caudal neuropore

Question 18

What happens if the rostral neuropore fails to close?

Answer 18

Anencephaly

- brain fails to develop

Question 19

What happens if the caudal neuropore fails to close?

Answer 19

Some forms of spina bifida

- neural tissue remain open to the environment

Question 20

Name cells which are derived from neural crest cells

Answer 20

\+ Cranial nerve ganglia
\+ Dorsal root ganglia
\+ Autonomin ganglia
\+ Adrenal medulla
\+ Schwann cells

+ Peripheral glial

+ Smooth muscle of cardiac flow

+ Odontoblasts
+ Craniofacial skeleton

+ Thyroid parafollicular (C) cells
+ Melanocytes

…

Question 21

What are two possible syndromes that can develop due to defective neural crest development?

Answer 21

+ Waardenburg’s Syndrome (1/50,000)

+ Treacher Collins Syndrome (autosomal dominant 1/50,000)

Question 22

What are features of Waardenburg’s Syndrome?

Answer 22

+ Some types have Pax-3 deletion

+ Pigment abnormalities (even albinism)

+ Deafness

+ Heterochromia of eyes

+ Telecanthus (widely serparated eye “corners”)

+ Other types show additional constipation (SOX10)

Question 23

What are features of Treacher Collins Syndrome?

Answer 23

+ Defective protein called Treacle (TCOF1 gene)

+ Failure of formation/apoptosis of neural crest cells

+ Abnormal eye shape

+ Micrognathia

+ Conductive heating loss

Underdeveloped zygoma

+ Malformed ears

Question 24

What are the different layers of mesoderm?

Answer 24

+ Paraxial
+ Intermediate
+ Lateral plate

Question 25

What is formed when the paraxial mesoderm becomes segmented?

Answer 25

Somites

Question 26

What are the features of somite formation?

Answer 26

+ Cells have a timer along paraxial mesoderm

+ Go through cycles every 90 mins defined by notch signalling clock

+ Wave of FGF signal that passes along the embryo

+ When wave passes cells they are programmed to change into part of a somite

Question 27

What are the crucial timings of the somite formation cycle?

Answer 27

+ If the wave passes cells early in the cycle they become the front end of the segment

+ If the wave passes cells late in the cycle they become the tail end of the segment

The process is repeated over and over

Question 28

What do homeobox genes do?

Answer 28

+ Direct formation of body structures e.g from body segments

+ Critical for forming body axes

Question 29

What are derivates of the paraxial mesoderm?

Answer 29

+ Head

+ Somites (sclerotome, myotome, dermatome)

Question 30

What are derivates of the intermediate mesoderm?

Answer 30

+ Kidneys

+ Gonads

Question 31

What are derivates of the lateral plate mesoderm?

Answer 31

+ Splanchnic (coverings, viscera, CVS)

+ Somatic (parietal, body wall)

Question 32

What are derivates of the midline mesoderm?

Answer 32

+ Prechordal plate

+ Notochord (nucleus pulposus)

Question 33

What does the yolk sac become?

Answer 33

The gut tube (remnant of gut tube)

Question 34

How does the gut tube form?

Answer 34

Amniotic cavity enwraps and surrounds the embryo, almost completely cutting off gut from the yolk sac

Question 35

What does the urinary bladder have a connection with?

Answer 35

The cloaca

Question 36

When does the vitelline duct usually obliterate?

Answer 36

Around 5-6 weeks

Question 37

What is and what causes Meckels diverticulum?

Answer 37

+ A congenital abnormality of the small intestine

+ Due to remnant of vitalline duct (yolk sac/gut connection)

Question 38

What are some derivatives of the ectoderm?

Answer 38

\+ Epidermis
\+ Lens and retina
\+ Olfactory epithelium
\+ Oral cavity epithelium
\+ Sensory organs of ear
\+ Glands: salivary, sweat, mammary
\+ Adenohypophysis